Convert standalone standby DB to a Standby RAC

We currently have a test environment that has a successful
implementation of DataGuard in fast_start failover mode. The primary DB
is a 2 node RAC and the physical standby is a standalone server. What we
want to do know is test both the primary and standby servers in RAC
mode.

Is it possible to convert a standalone server to RAC once the standalone
has been setup as a physical standby server? Do we have to drop the
standalone DB and set it up as RAC and then convert it back to a standby
server? I've looked at the documentation and read how to convert a
standalone DB to a RAC db but it doesn't describe how it should be done
if the standalone server is in a dataguard setup.